GCP uses Nvidia but Stadia uses AMD GPUs so they're not sharing hardware.
It could also be a way for AMD to get a ton of GPUs sold after oversupply due to the mining boom/bust.
The GPUs are custom for Google.
Do we know custom how? It might be the same chips with different microcode or whatever GPUs' equivalent is. Is it a custom spin like Intel has been doing for years for Google and Amazon?